There exists the need for low purity oxygen in many industrial processes, for example coal and petcoke gasification. For such processes, often an enclave of such plants is provided at a single location to supply the necessary low purity oxygen. In integrated gasification combined cycles, high pressure nitrogen is often required as a product that is used in enhancing the power of gas turbines and for NOx control. In some locations, there is also a desire to produce chemicals in addition to power generation from the gasification process. In such locations, there exists the need to produce a high pressure, high purity gaseous oxygen product. Moreover, there may also be a demand for argon.
Where oxygen is required for such purposes as gasification, the most practical way to produce the oxygen is by means of the cryogenic rectification of air. In such a process, the incoming air is compressed, purified and then cooled to a temperature suitable for its rectification within a main heat exchanger. The resulting compressed, cooled and purified air is then introduced into an air separation unit that typically consists of high and low pressure columns. In the high pressure column, the air is rectified to produce a nitrogen-rich column overhead. At least a portion of such column overhead is condensed to produce reflux to both the high and low pressure columns. An oxygen-rich column bottoms is produced within the high pressure column that is known as kettle liquid or crude liquid oxygen. A stream of such bottoms liquid is introduced into the low pressure column for further refinement. As a result of such further refinement, an oxygen-rich liquid column bottoms is produced in the low pressure column that can be taken as an oxygen-rich product.
An example of an air separation plant that can be used in the generation of low purity oxygen is disclosed in U.S. Pat. No. 5,675,977. In the plant shown in this patent, the nitrogen-rich vapor produced in the higher pressure column is in part condensed in a bottom reboiler located in the base of the lower pressure column to generate liquid reflux streams that are used to reflux both the higher and lower pressure columns. Another part of the nitrogen-rich vapor is taken as a high pressure product that is fully warmed in the main heat exchanger. A nitrogen product stream can also be taken at a lower pressure from the top of the low pressure column and fully warmed to produce a low pressure nitrogen product. A stream of oxygen-rich liquid is taken from the base of the low pressure column and optionally pumped and vaporized in the main heat exchanger to produce a high pressure oxygen product having a low purity. In order to generate sufficient reflux to enable production of the high pressure nitrogen product, the crude liquid oxygen or kettle liquid is taken as a stream and introduced into an auxiliary kettle liquid column for rectification. Nitrogen containing vapor from the top of the auxiliary column is used in reboiling the low pressure column at an intermediate point to generate liquid that is used in the reflux of both the auxiliary column and the low pressure column.
There are a variety of cryogenic air separation plants that are designed to produce both a low purity oxygen product and a higher purity oxygen product. For example, in U.S. Pat. No. 5,628,207, oxygen-rich column bottoms of the low pressure column is pumped and then introduced into an auxiliary column. This column is reboiled by compressing and cooling a portion of the nitrogen-rich vapor column overhead produced in the high pressure column. The resulting residual liquid is the ultra-high purity liquid oxygen that can be taken as a product. A gaseous stream can be removed from the top of the column and fully warmed to produce the low purity oxygen product.
It is to be noted that the air separation plant shown in U.S. Pat. No. 5,628,207, is a highly integrated plant in which all of the low purity oxygen is pumped and introduced into the auxiliary column for vaporization and for separation to produce the high purity liquid oxygen. Thus, although one plant in an enclave could be constructed using the teachings of this patent, it is not very amenable for a retrofit situation. Additionally, since all of the low purity oxygen passes through the auxiliary column, there is no way to provide an original installation where there exists a low requirement for high purity liquid oxygen and potentially argon.
As will be discussed, the present invention provides a method and apparatus for separating air that is more flexible in its production of high purity liquid oxygen and that is more amendable to the prior art in integrating such production with an existing enclave of cryogenic air separation plants. Moreover, the present invention allows argon contained in the low purity oxygen to be recovered.